The present invention relates to an aqueous slurry for use in chemical-mechanical polishing processes. More particularly, the present invention relates to an aqueous slurry that is particularly useful for selectively removing silicon dioxide in preference to silicone nitride from a surface of an article by chemical-mechanical polishing, and a method of removing silicon dioxide in preference to silicon nitride from a surface of an article by chemical-mechanical polishing.
Chemical-mechanical polishing (xe2x80x9cCMPxe2x80x9d) is a technology which has its roots in the pre-industrial era. In recent years, CMP has become the technology of choice among semiconductor chip fabricators to planarize the surface of semiconductor chips as circuit pattern layers are laid down. CMP technology is well-known, and is typically accomplished using a polishing pad and a slurry which contains a chemical reagent and abrasive particles. The chemical reagent functions to chemically react with the surface of the layer being polished whereas the abrasive particles perform a mechanical grinding function.
One of the uses of CMP technology is in the manufacture of shallow trench isolation (STI) structures in integrated circuits formed on semiconductor chips or wafers such as silicon. The purpose of an STI structure is to isolate discrete device elements (e.g., transistors) in a given pattern layer to prevent current leakage from occurring between them. Recent technological advancements permitting the fabrication of very small, high density circuit patterns on integrated circuits have placed higher demands on isolation structures.
An STI structure is usually formed by thermally growing an oxide layer on a silicon substrate and then depositing a silicon nitride layer on the thermally grown oxide layer. After deposition of the silicon nitride layer, a shallow trench is formed through the silicon nitride layer and the thermally grown oxide layer and partially through the silicon substrate using, for example, any of the well-known photolithography mask and etching processes. A layer of a dielectric material such as silicon dioxide is then typically deposited using a chemical vapor deposition process to completely fill the trench and cover the silicon nitride layer. Next, a CMP process is used to remove that portion of the silicon dioxide layer covering the silicon nitride layer and to planarize the entire surface of the article. The silicon nitride layer is intended to function as a polishing stop that protects the underlying thermally grown oxide layer and silicon substrate from being exposed during CMP processing. In some applications, the silicon nitride layer is later removed by, for example, dipping the article in an HF acid solution, leaving only the silicon dioxide filled trench to serve as an STI structure. Additional processing is usually then performed to form polysilicon gate structures.
It should be readily apparent that during the CMP step of manufacturing an STI structure on a silicon semiconductor substrate, it would be highly advantageous to use a polishing agent that is capable of selectively removing silicon dioxide in preference to silicone nitride, which is used as the stop layer. Ideally, the rate for removing silicon nitride by CMP would be close to 0 whereas the rate for removing silicon dioxide by CMP would be as high as possible.
Throughout the specification and in the appended claims, the term xe2x80x9cselectivityxe2x80x9d is used to describe the ratio of the rate at which silicon dioxide is removed to the rate at which silicon nitride is removed by the same polishing agent during a CMP process. Selectivity is determined by dividing the rate at which the silicon dioxide film is removed (usually expressed in terms of mu/min) by the rate at which the silicon nitride film is removed. Conventional CMP slurries occasionally exhibit a silicon dioxide to silicon nitride selectivity of up to about 10, and typically of about 4 or 5.
Keeping the silicon dioxide to silicon nitride selectivity high for a CMP slurry is important. However, suppressing the rate of removal of silicon nitride is probably more important. It is known that the removal rate of the silicon dioxide trench fill material can be made to be quite high by varying polishing conditions such as increasing pad pressure and using various abrasive particles in the slurry. However, these polishing conditions also tend to increase the silicon nitride removal rate, which can affect the uniformity of the final silicon nitride layer thickness and can cause other defects, such as scratching, in the final product. Thus, it is important for a CMP slurry to promote a reasonable silicon dioxide removal rate while, at the same time, inhibiting or suppressing the rate of silicon nitride removal. This too, however, must be done in moderation for some applications. When the selectivity of a CMP slurry is too high coupled with a very low silicon nitride removal rate, other problems, such as dishing of the trench silicon dioxide, can occur, which can result in severe topography variations once the silicon nitride stop layer is removed. Thus, an aqueous slurry should be able to balance these factors in order to be useful in STI processing.
One of the factors that can affect the polishing rate during a CMP process is the pH of the slurry. For some CMP slurries, a slight change in pH can result in a substantial difference in polishing rate and selectivity. It is preferably for a CMP slurry to exhibit a relatively high polishing rate and high selectivity over a broad range of pH.
Most conventional CMP slurries used for polishing oxides typically comprise abrasive particles dispersed in an aqueous alkaline medium (i.e., high pH). Such slurries tend to polish silicon dioxide and silicone nitride at a substantial rate, with a selectivity for silicon dioxide of about 10 or below, and typically at about 4. A few CMP slurries are known which do provide a fairly high silicon dioxide to silicon nitride removal rate selectivity. However, none of the heretofore known CMP slurries exhibit high silicon dioxide to silicon nitride selectivity over a broad range of pH.
Hosali et al., U.S. Pat. No. 5,738,800, discloses a composition for polishing a composite comprised of silicon dioxide and silicon nitride. The CMP slurry according to Hosali et al. comprises an aqueous medium, abrasive particles, a surfactant, and a complexing agent having two or more functional groups each having a dissociable proton that complexes with the silicon dioxide and silicon nitride. The surfactant used in conjunction with the complexing agent in the CMP slurry according to Hosali et al. does not perform the usual function of surfactants (i.e., the stabilization of the particulate dispersion), but rather it is believed by the inventors to affect the rate of removal of silicon nitride from the composite surface. The chemistry of the interaction between the surfactant and the complexing agent is not explained. The composition according to Hosali et al. exhibits selectivity better than conventional CMP slurries, but only within a narrow range of pH (from about 6 to about 7).
Grover et al., U.S. Pat. No. 5,759,917, discloses a CMP slurry for selectively polishing a silicon dioxide overfill in preference to a silicon nitride film stop layer during the manufacture of integrated circuit""s and semiconductors. The CMP slurry according to Grover et al. comprises a carboxylic acid, a salt, and a soluble cerium compound at a pH within the range of from about 3 to about 11. The specification of Grover et al. states that a silicon dioxide to silicon nitride removal selectivity of from about 5 to about 100 is obtainable, but the highest reported selectivity in any of the examples in Grover is 34.89, and the substantial majority of the examples yield a selectivity of less than 20.
Kodama et al, EP 0 786 504 A2, discloses a CMP polishing composition comprising silicon nitride particles, water, and an acid. The CMP polishing composition according to Kodama et al. is said to exhibit high selectivity for polishing silicon dioxide relative to silicon nitride. The highest selectivity reported in any of the examples in Kodama et al. is 32.5, and the substantial majority of the examples yield a selectivity of less than 20.
Ronay, EP 0 846 740 A1, discloses a CMP slurry for STI that contains abrasive particles and a polyelectrolyte having a molecular weight of between about 500 and 10,000 such as, for example, polyethylenimine. The pH of the slurry must be kept between 9-11, and there is no information provided regarding whether the CMU slurry according to Ronay provides any degree of selectivity between silicon dioxide and silicon nitride.
Morrison et al., EP 0 853 335 A2, discloses a CMP slurry for STI processing that comprises a mixture of a conventional CMP slurry (typically colloidal silica suspended in an aqueous medium) to which has been added tetramethyl ammonium hydroxide and hydrogen peroxide. The modified CMP slurry according to Morrison et al. is said to improve the typical silicon dioxide to silicon nitride selectivity of 4 to as high as 30. The pH of the slurry must be maintained within a rather narrow range of from about 11 to 12.9.
Several literature references also discuss CMP slurries for use in STI processing. For example, Chemical Mechanical Planarization of Microelectronic Materials, by J. M. Steigerwald, S. P. Muraka, and R. J. Gutman, ISBN 0-471-13827-4 (Jon Wiley and Son Inc. 1997), generally discusses the desirability of using STI processing instead of Local Oxidation of Silicon (LOCOS) processing. While this reference teaches that conventional CMP processes have low selectivity between oxides and nitrides, no insight into a potential solution for this problem is taught.
Another literature reference, A High Oxide:Nitride Selectivity CMP Slurry for Shallow Trench Isolation, by Sharath Hosali and Ray Lavoie, in Electromechanical Society Proceedings Volume 98-7 (1998), pages 218-234, discloses a slurry that is said to enhance the selectivity rate between silicon oxide and silicon nitride removal by CMP processes. The slurry disclosed in that reference includes cerium oxide as an abrasive with an undisclosed proprietary solution that inhibits the removal rate of the silicon nitride. This references reports that a high rate of selectivity can be obtained for unpatterned silicon wafers. However, the selectivity for patterned silicon wafers is reported to be almost the same as with the conventional CMP slurry it was compared against.
Another literature reference, Application of Ceria-based High Selectivity Slurry to STI CMP For Sub 0.18 xcexcm CMOS Technologies, by Ki-Sik Choi, Sang-Ick Lee, Chang-Il Kim, Chul-Woo Nam, Sam-Dong Kim, and Chung-Tae Kim, CMP-MIC Conference, Feb. 11-12, 1999, pages 307-313, discloses the use of a ceria-based CMP slurry in the process of forming an STI structure, but no specific information is provided regarding how to prepare the slurry. This reference teaches that dummy patterns are required in order to minimize a phenomena known as dishing, which is the formation of shallow depressions in the silicon dioxide filled trenches below the plane of the top surface of silicon nitride stop layer during CMP processing. This reference also reported that there were some problems related to scratches being formed by the ceria abrasive which required filtering to correct.
Yet another literature reference, A Production-Proven Shallow Trench Isolation (STI) Solution Using Novel CMP Concepts, by Raymond R. Jin, Jeffery David, Bob Abbassi, Tom Osterheld, and Fritz Redeker, CMP-MIC Conference on Feb. 11-12, 1999, pages 314-321, discusses the problem of having to use dummy patterns to reduce dishing. The solution offered is to employ a low selectivity, or no selectivity, CMP slurry for minimizing dishing during CMP processing in combination with a special system, apparatus, and polishing heads.
Finally, yet another literature reference, A Wide Margin CMP and Clean Process For Shallow Trench Isolation Applications, by Brad Withers, Eugene Zhoa, Rahul Jairath, CMP-MIC Conference on Feb. 19-20, 1998, pages 319-327, addresses the problems of process cost and complexity due to the need for block masks, pattern resist etch, high selectivity material overlays or implementation of dummy active areas. No solutions for these problems are provided.
It should be readily apparent from the foregoing discussion that there remains a need within the art for a method of chemical mechanical polishing and a slurry that exhibits high selectivity for silicon dioxide in preference to silicon nitride and that has a wide working range of pH.
The present invention provides a novel aqueous slurry that is effective over a wide range of pH for selectively removing silicon dioxide in preference to silicon nitride from a surface of an article by chemical-mechanical polishing. The aqueous slurry according to the present invention comprises abrasive particles and an organic compound having both a carboxylic acid functional group and a second functional group selected from amines and halides. In a preferred embodiment, the abrasive particles comprise ceria or titania and the organic compound comprises an a-amino acid such as, for example, proline, glycine, or alanine. The aqueous slurry can optionally include acids or bases for adjusting the pH within an effective range of from about 4 to about 12.
The present invention also provides a novel method of selectively removing silicon dioxide in preference to silicon nitride from a surface of an article by chemical-mechanical polishing. The method of the present invention comprises polishing a surface of an article using a polishing pad, water, abrasive particles, and an organic compound having both a carboxylic acid functional group and a second functional group selected from amines and halides. The abrasive particles can be dispersed in the aqueous medium or they can be bonded to the polishing pad.
The novel aqueous slurry and method of the present invention are particularly useful in applications such as, for example, the fabrication of STI structures, where precise control over the silicon dioxide to silicon nitride polishing rate selectivity is critical. The organic compound having both a carboxylic acid functional group and a second functional group selected from amines and halides functions to reduce the rate of silicon nitride removal without significantly reducing the rate of silicon dioxide removal. The removal rate of silicon nitride can be controlled by varying the concentration of the organic compound in the slurry, whereas the removal rate of silicon dioxide can be regulated by changing other CMP conditions, such as abrasive concentration and size, polishing pad down force, and/or speed. Silicon dioxide to silicon nitride removal selectivity can be adjusted within a working range from as low as about 5 to as high as about 500 or more. Many of the compounds used in the aqueous slurry and method according to the present invention are environmentally benign, and thus present minimal considerations in terms of waste water treatment and disposal.